Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against wakeup

["Kohli, Gaurav" <gkohli@codeaurora.org>]

On 5/1/2018 5:01 PM, Peter Zijlstra wrote:
> On Tue, May 01, 2018 at 04:10:53PM +0530, Kohli, Gaurav wrote:
>> Yes with loop, it will reset TASK_PARKED but that is not happening in the
>> dumps we have seen.
> 
> But was that with or without the fixed wait-loop? I don't care about
> stuff you might have seen with the current code, that is clearly broken.
> 
>>> takedown_cpu() can proceed beyond smpboot_park_threads() and kill the
>>> CPU before any of the threads are parked -- per having the complete()
>>> before hitting schedule().
>>>
>>> And, afaict, that is harmless. When we go offline, sched_cpu_dying() ->
>>> migrate_tasks() will migrate any still runnable threads off the cpu.
>>> But because at this point the thread must be in the PARKED wait-loop, it
>>> will hit schedule() and go to sleep eventually.
>>>
>>> Also note that kthread_unpark() does __kthread_bind() to rebind the
>>> threads.
>>>
>>> Aaaah... I think I've spotted a problem there. We clear SHOULD_PARK
>>> before we rebind, so if the thread lost the first PARKED store,
>>> does the completion, gets migrated, cycles through the loop and now
>>> observes !SHOULD_PARK and bails the wait-loop, then __kthread_bind()
>>> will forever wait.
>>>
>>
>> So during next unpark
>> __kthread_unpark -> __kthread_bind -> wait_task_inactive (this got failed,
>> as current state is running so failed on below call:
> 
> Aah, yes, I seem to have mis-remembered how wait_task_inactive() works.
> And it is indeed still a problem..
> 
> Let me ponder what the best solution is, it's a bit of a mess.
> 

Sure , Thanks a lot.
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